Tuesday, June 16, 2009

Why are there so many overtime games in the NBA?

There are a lot of NBA games that end up tied after 48 minutes – almost twice as many as expected.

A post from the "Cheap Talk" blog charted score differences for every NBA game between 1997 and 2009. It found a fairly smooth curve, except at zero, where that particular outcome was about twice as frequent as expected.

As it turns out, the spike comes only in the last few seconds of the game. As the authors show in a video on their post, there is barely any spike at all with 40 seconds left. The spike starts to emerge at about 20 seconds, and then grows steadily until 0:00.

Why does this happen? The authors don't really give a hypothesis. One obvious reason, though, is that for the team that's behind, closing the deficit is worthless unless they tie or take the lead. In the first quarter, a team down by three might go for the easy two-pointer instead of the unlikely three. But, with five seconds to go, only the three has any value. Therefore, it's either a tie or nothing.

And if you look at the video again, you'll see that it's not just that zero spikes, but that the values surrounding zero drop. That makes sense – if the team behind by three fails to make it, they'll start fouling the opposition. The most likely result is that they fall further behind. So you get a spike at zero, but a drop at (say) –3, because a minus three with five seconds left gets turned into a minus six or something.

Another possibility (as mentioned by commenters to the post) is that teams are overly conservative. With a tie game and five seconds left, the team with possession might decide to run out the clock instead of risking a turnover. Or, a team behind by two might go for the field goal instead of the three, even if the three is the better strategy. Consider two equally-matched teams. A 40% chance to make two points (and tie) gives you a 20% chance of winning the game. A 30% chance to make a three (and win) gives you a 30% chance of winning the game. A conservative coach might go for the two anyway.

That possibility needs more investigation; I wouldn't accuse teams of playing suboptimally without a bit more evidence. I have to admit, though, that it does have some intuitive plausibility.